Saturday, October 3, 2009

Trees with opposite leaves

In the past, tree identification guides have emphasized the presence of opposite leaves as one of the most important field marks. In the Sibley Guide to Trees I used a more holistic approach, like modern bird identification, giving equal weight to all parts of the tree. A tree might catch your attention because of its flowers, twigs, bark, fruit, etc., and I don’t think it’s helpful if the field guide then asks you to check whether the leaves are alternate or opposite. It’s better to get to know all of the distinguishing characteristics of oaks and maples, for example, so that you can recognize them by multiple features, rather than having to check leaf arrangement.

In some cases, however, when you encounter a confusing tree, the presence of opposite leaves can be a useful pointer to help narrow the range of possibilities. So below I’ve put together a list of trees with opposite leaves. Interestingly, almost all of these species show up in the last 80 pages of the guide.

Definition from p xxiv of The Sibley Guide to Trees:

Leaves that grow singly from the twig, without another leaf directly across the twig, are called alternate, as they usually grow from alternate sides of the twig. Leaves that grow in pairs from opposite sides of the twig at the same level are called opposite. If three or more leaves all grow from the same level on the twig they are called whorled.

These patterns can be detected even more easily in winter, as buds and leaf scars on the twigs match the arrangement of the leaves (either opposite or alternate), and branching tends to be opposite or alternate as well. Also note that twigs with alternate leaves tend to grow in a zigzag pattern, while trees with opposite leaves generally have straighter twigs.

Species counts only include those illustrated in the guide.

North American Trees with Opposite Leaves

Katsura, Genus Cercidophyllum (p. 101) – one introduced species [the only species in its family]

Eucalyptus, Genus Eucalyptus (p 109) – three introduced species have juvenile foliage opposite, but adult foliage alternate [Crape-myrtle, in the same family, has alternate leaves]

Paper Mulberry, Genus Brousonettia (p. 248) – one introduced species [other genera in the Mulberry family do not have opposite leaves]

Lyontree, Genus Lyonothamnus (p. 293) – one native species [other genera in the Rose family do not have opposite leaves]

Beebee Tree, Genus Evodia (p. 321) – one introduced species [related to Corktree, but other species in the Rue family do not have opposite leaves]

Corktree, Genus Phellodendron (p. 323) – one introduced species [related to Beebee Tree, but other species in the Rue family do not have opposite leaves]

Buckeyes, Genus Aesculus (p. 326-331) – six native and one introduced species [related to maples, but other genera in the Soapberry family do not have opposite leaves]

Maples, Genus Acer (p. 332-351) – 13 native and ten introduced species [related to buckeyes, but other genera in the Soapberry family do not have opposite leaves]

Dogwoods, Genus Cornus (p. 355-358) – three (out of four) native and one introduced species have opposite leaves [Dove Tree, in the same family, does not]

Buttonbush, Genus Cephalanthus (p. 374) – one native species [the only North American tree in the Madder family]

Ash, Genus Fraxinus (p. 376-385) – 17 native and four introduced species [all other genera in the Olive family also have opposite leaves]

Japanese Tree-lilac, Genus Syringa (p. 386) – one introduced species [all other genera in the Olive family also have opposite leaves]

American Fringetree, Genus Chionanthus (p. 387) – one native species [all other genera in the Olive family also have opposite leaves]

Common Olive, Genus Olea (p. 388) – one introduced species [all other genera in the Olive family also have opposite leaves]

Devilwood, Genus Osmanthus (p. 388) – one native species [all other genera in the Olive family also have opposite leaves]

Desert-willow, Genus Chilopsis (p. 389) – one native species with opposite or whorled leaves [related to Catalpas, other genera in the Trumpet Creeper family also have opposite leaves]

Catalpas, Genus Catalpa (p. 390-391) – two native and one introduced species have opposite or whorled leaves [related to Desert-willow, other genera in the Trumpet Creeper family also have opposite leaves]

Royal Paulownia, Genus Paulownia (p. 392) – one introduced species [the only species in the family]

Elders, Genus Sambucus (p. 397) – many native species mostly shrubs [related to Viburnums, all other genera in the Muskroot family have opposite leaves]

Viburnum, Genus Viburnum (p. 397) – many native species mostly shrubs [related to Elders, all other genera in the Muskroot family have opposite leaves]

Hi Kevin, These species have alternate leaves, but I see where the confusion comes from. The illustrations of leaves of pistachios and peppertrees in the guide show single compound leaves. So what you are seeing are opposite leaflets on a leaf, but these compound leaves would be arranged alternately along the twigs.

A neophyte error as (clearly) all of your illustrations are of individual leaves. And you do show a Peppertree twig with alternate compound leaves.

I think I got that from thinking about ashes with opposite leaves but, of course, it's the compound leaves that are opposite to each other (as well as being "opposite" in their construction from "subleaves") .

The Cork Tree threw me for years: opposite leaves, but I just couldn't find it in any of my guide books. Yet we saw it constantly at one of our local nature centers. Then I realized it wasn't a native! Learning that not all trees are native has been a lengthy learning process.......

Still don't have your new guide but my birthday is coming up soon and it's up near the top of the wish list! I'm sure that the inclusion of non-native trees will be very helpful to many people.